1. Field of the Invention
The invention relates in general to an emission layer and an organic light emitting diode using thereof, and more particularly to an emission layer and an organic light emitting diode using thereof for emitting a white light.
2. Description of the Related Art
A conventional organic light emitting diode (OLED) is a multi-layer structure including a substrate, an anode, a hole-transport layer, an emission layer, an electron-transport layer, and a cathode. The anode, the hole-transport layer, the emission layer, the electron-transport layer, and the cathode are disposed on the substrate orderly from bottom to top. The emission layer includes a host-dopant system, in which a few dopants are doped in host material.
When a voltage is applied to the cathode and the anode, electrons are injected into the emission layer from the cathode through the electron-transport layer. Holes are injected into the emission layer from the anode and through the hole-transport layer. After the electrons and the holes combine in the emission layer, the host material is excited from a stable state to an excited state. Because the host material in the excited state is unstable, the host material certainly returns to the stable state and transfer energy to the dopants.
When the dopants receiving the energy are excited from the stable state to the excited state, singlet excitons and triplet excitons are generated by the dopants. Due to the distribution ratio of the electron spin state, the probability of forming the triplet excitons and the singlet excitons is approximately 3:1. Electroluminescence occurs in the OLED while the singlet excitons or the triplet excitons return to the stable state by releasing photons.
Combined by two or three light-emitting materials that generate lights of different wavelength, the emission layer emits white light. For example, in addition to blue, green, and red light-emitting materials, there is also a combination of blue-green, yellow and orange light-emitting materials in order to formulate white-light emitting diode. Further, the OLED can emit white light by emitting a combination of two colors, such as blue-green light and red light or a combination of blue light and orange light. The conventional emission layer is classified into three kinds by its structure. The first kind of the emission layer has three sublayers relatively comprising red, green, and blue light emitting material doped in the host material. Each sublayers emits red, green or blue light in order to formulate white light diode. The second kind of the emission layer has two sublayers relatively comprising blue and red light emitting material doped in the host material. The third kind of the emission layer has only one layer comprising yellow or red light emitting material doped in the blue host material.
However, the emission layer with two or three sublayers provides high luminance efficiencies combined with high color purity and long lifetimes, but the process for manufacturing is too complicated. On the other hand, the single one emission layer is rather easy to manufacture, but provides low luminance efficiency and poor color purity. Notwithstanding these developments, there are continuing needs for OLED components, such as light-emitting materials, sometimes referred to as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes and simplified manufacture process.